Adaptive immunity is defined by two important characteristics: specificity and memory. Specificity refers to the adaptive immune system’s ability to target specific pathogens, and memory refers to its ability to quickly respond to pathogens to which it has previously been exposed. For example, when an individual recovers from chickenpox, the body develops a memory of the infection that will specifically protect it from the virus if it is exposed to it again later. 

Specificity and memory are achieved by essentially programming certain cells involved in the immune system to respond rapidly to subsequent exposures of the pathogen. This programming occurs as a result of the first exposure to a pathogen, which triggers a primary response. Later exposures result in a secondary response that is faster and stronger as a result of the body’s memory of the first exposure. This secondary response, however, is specific to the pathogen in question. For example, exposure to one virus, like the chickenpox, will not provide protection against other viral diseases.